The present invention relates generally to chip to chip communications, and, particularly to matching the input impedance of a receiver circuit in a first chip with the output impedance of a driver circuit in a second chip and with the characteristic impedance of a transmission line coupling the driver circuit to the receiver circuit.
As signal rates increase in integrated circuit (chip) technology, many of the chip to chip communications require matched impedances between the driver circuits and the receiver circuits on chips. This is required in order to achieve maximum data transfer rates between a driver circuit and a receiver circuit and to reduce reflection of data signals in the transmission line which couples the driver circuit to the receiver circuit.
An ideal driver-receiver system would consist of a remote driver circuit, a transmission line, and a receiver circuit. In such an ideal driver-receiver system, the driver circuit is a source pulse generator which has a Thevenin equivalent output impedance, Z.sub.Do, matching the characteristic impedance, Z.sub.To, of the transmission line and matching the input impedance, Z.sub.Ro, of the receiver circuit.
Realistically, a non-ideal driver-receiver system implementing chip to chip communication includes a remote driver circuit, a transmission line, an impedance matching circuit, and a receiver circuit. The impedance matching circuit matches the input impedance of a receiver circuit with the output impedance of the remote driver circuit and with the characteristic impedance of the transmission line.
For example, FIG. 1 shows a block diagram of a driver-receiver system 100 which includes an impedance matching circuit 130. Driver-receiver system 100 includes a remote driver circuit 110 coupled to transmission line 120, which is coupled to impedance matching circuit 130, which in turn is coupled to receiver circuit 140. Remote driver circuit 110 receives an original data signal, A, and outputs a driver data signal, A.sub.D, to transmission line 120. Transmission line 120 transmits driver data signal A.sub.D and outputs transmission data signal A.sub.T to impedance matching circuit 130. Impedance matching circuit 130 outputs an impedance-matched data signal, A.sub.IM, to receiver circuit 140. Receiver circuit 140 outputs a receiver data signal, A.sub.R.
FIG. 2A shows a first type of known driver-receiver system 210 which includes a first type of known impedance matching circuit 130. This first type of known impedance matching circuit 130 includes a first resistor 220, a second resistor 230, and an inverter 240. In one case, first resistor 220 and second resistor 230 are external termination resistors which are discrete resistors added to the printed circuit board (PCB) on which the chip with remote driver circuit 110 and the chip with receiver circuit 140 are mounted. In another case, first resistor 220 and second resistor 230 are fabricated into the package that supports the chip with receiver circuit 140 and make the electrical connections available to the PCB wires. In both cases, the parallel combination of the resistances of first resistor 220 and second resistor 230 is set to generate an input impedance for receiver circuit 140 which matches the output impedance of remote driver circuit 110 and the characteristic impedance of transmission line 120. However, system 210 which includes impedance matching circuit 130 poses several problems. For example, system 210 requires additional work for mounting first resistor 220 and second resistor 230 either on the PCB or in the package which supports the chip with receiver circuit 140. In addition, system 210 less reliably maintains the input impedance for receiver circuit 140 constant because of the external connections required between the chip with receiver circuit 140 and either the PCB or the package which supports the chip with receiver circuit 140.
Other types of known driver receiver systems exist which are similar to the first type of known driver-receiver system 210. For example, instead of using discrete resistors to generate the input impedance of receiver circuit 140, bipolar transistors are used to generate the input impedance of receiver circuit 140. In that case, the bipolar transistors are configured as resistors and take the place of first resistor 220 and second resistor 230 in system 210. In another example, PMOS transistors configured as resistors are used. Both of these alternative versions pose several problems First, they do not effectively maintain the input impedance of receiver circuit 140 constant over temperature. Also, they do not effectively compensate for process variations in the manufacturing of the chips which the impedance matching circuit 130 is supposed to interact with.
FIG. 2B shows a second type of known driver-receiver system 250 which includes a second type of known impedance matching circuit 130. This second type of known impedance matching circuit 130 includes a first transistor 260, a second transistor 270, and an inverter 280. A control signal biases first transistor 260 such that the parallel combination of first transistor 260 and second transistor 270 generates an input impedance for receiver circuit 140 which matches the output impedance of remote driver circuit 110 and the characteristic impedance of transmission line 120. However, system 250 also is subject to problems with temperature and process variations.
For the foregoing reasons, an impedance matching circuit which maintains the input impedance of the receiver circuit constant over temperature variations and over process variations, without the use of external resistors, would greatly benefit chip to chip communications.